1. Field of the Invention
The invention relates to a charging device, and more particularly to a charging device for a flash capacitor.
2. Description of the Related Art
In general, a camera comprises a flash module, a flash capacitor and a charging device for charging the flash capacitor. The flash capacitor will store energy in advance, so as to provide the stored energy to the flash capacitor to generate a flash when a user uses a flash function to take a picture.
FIG. 1 shows a conventional capacitor charging circuit 100 disclosed in U.S. Pat. No. 7,292,005. The capacitor charging circuit 100 comprises a control circuit 110, a measuring circuit 120 and a power delivery circuit 130, wherein the power delivery circuit 130 is coupled to an output capacitor 150 via an output diode 140. In the power delivery circuit 130, an OR gate 132 receives a first signal from a determining circuit 131 and a second signal from the control circuit 100, and generates a first control signal to a latch 133 to turn on a transistor 134. In addition, the latch 133 turns the transistor 134 off according to a second control signal provided by a determining circuit 135.
In FIG. 1, an input source 160 provides energy to a transformer 170 when the transistor 134 is turned on. Next, the transformer 170 transforms the energy stored at a primary winding to a secondary winding. Furthermore, when the secondary voltage of the transformer 170 is increased, the primary voltage of the transformer 170 is also increased. By switching the transistor 134 continuously, the capacitor charging circuit 100 charges the output capacitor 150 to a desired voltage.
FIG. 2A and FIG. 2B separately show operation of the transistor 134 at the nth switch state and the mth switch state (m>n). The signal SC represents the switch signal provide by the latch 133, which is used to control the transistor 134 to turn on or off, and the signal VSW represents the primary voltage of the transformer 170. When the secondary voltage of the transformer 170 is output to the output capacitor 150 (i.e. the output capacitor 150 is charged), the secondary voltage is decreased, and then the primary voltage VSW of the transformer 170 is also decreased. As shown in FIG. 2A and FIG. 2B, when the primary voltage VSW received by the comparator 136 is smaller than a reference voltage VREF) the latch 133 turns the transistor 134 on according to the first signal provide by the determining circuit 131. For example, in FIG. 2A, the transistor 134 is turned on when the primary voltage VSW is decreased from a voltage level VL1 to a voltage level of the reference voltage VREF. In FIG. 2B, the transistor 134 is also turned on when the primary voltage VSW is decreased from a voltage level VL2 to the voltage level of the reference voltage VREF, wherein VL2>VL1. When the primary voltage VSW, which is varied, following the secondary voltage of the transformer 170 is gradually increased, it takes an excess amount of time to decrease under the reference voltage VREF since the reference voltage VREF is fixed. For example, a response time period t2 of FIG. 2B is larger than a response time period t1 of FIG. 2A. Thus, charging time of the output capacitor 150 is increased and charging efficiency is decreased.
Therefore, a charging device which is able to speedily charge a flash capacitor is desired.